Miniature VHF-UHF portable radio receivers commonly use antennas of loop design arranged either as a single loop around the radio housing or as an internal loop concealed inside the radio housing. U.S. Pat. No. 3,736,591 issued to Rennels, et al, is an example of a loop antenna. The loop antennas are economical to manufacture and may be efficiently integrated into the radio package. However, their lack of omni-directionality generally limits their application to receivers used for personal communication such as, for example, paging devices and personal radios. The lack of omni-directional capability is acceptable in these applications as the user normally has freedom to reorient the radio to optimize reception.
Adapting these antenna designs to inventory tag application has not been successful since a tag on a stored or warehoused item maintains a fixed location and position. That is, its tag is unable to freely reorient itself to improve reception. Tags are installed on packages which are placed in warehouses, shipping containers or pallets and where the emphasis is on efficient use of storage space and not on optimizing radio communications. In addition if the package is moved, a tag that was in can an optimal communication position may be placed in a less favorable orientation. Since the tag is immobile, the lack of an omni-directional antenna greatly affects the utility of the tag. Also, in many circumstances, tags may be placed in "blind spots", where communication using conventional loop antennas is difficult due to shadowing or other obstructive influences.
To address this problem, radio tags used for inventory control ideally incorporate omni-directional antennas which provide good communications without regard to orientation. One such antenna is described in U.S. Pat. No. 5,485,166, "An Efficient Electrically Small Loop Antenna With A Planar Base Element" issued to Verma et al and assigned to the same assignee as the present invention. This patent discloses a loop antenna on a planar base element that has been shown to provide extremely efficient omni-directional communications. However, the three dimensional design of the antenna makes it unsuitable for many applications. Using such loop antennas (and similar omni-directional designs) for inventory tag applications requires a physical design that has substantial measurements in three dimensions. Applications involving tagging of large or non-uniform objects (for example, a vehicle) are served well by these existing designs since the tag adds little to the bulk of the object being tagged. For small or tightly packed items, however, these tag designs are not well suited as they simply take up too much space.
Slot antennas generally would appear to be a good choice for radio frequency inventory tag applications. Slot antennas are commonly used in high frequency radio bands (greater than 1 GHz) and typically feature slot lengths on the order of one-half wave length (1/2 .lambda.), which at these frequencies is approximately 6 inches. While such dimensions may be acceptable in physically large applications such as aircraft communications where the antenna fits within the skin of the aircraft, they are not suitable for a small portable receiver. To use conventional slot antenna designs in the VHF-UHF band (approximately 500 MHz), the 1/2 .lambda. slot length would be on the order of 12 inches. Thus, dimension requirements generally preclude conventional slot antennas from being used in inventory tag applications.
As discussed above, one of the necessary characteristics of an antenna intended for radio frequency inventory tag applications is omni-directionality. That is, the tag transceiver must be capable of good operation in any orientation. Prior art slot antennas developed for omni-directional pattern employ either a circular array or a cross-slot arrangement of the elements, the antenna being excited or driven either from a cavity underneath the antenna or from sources isolated from the antenna. This form of antenna design is undesirable for a radio frequency inventory tag application due to the size of the cavity or external placement of the drive means.
Designs in the prior art for miniature receivers have used slot type antennas; however, these have been formed on the edge of a thin receiver housing. For example, see U.S. Pat. No. 4,935,745 issued to Mori, et al. Such designs, although claiming efficiency of packaging, are sensitive to nearby structures or bodies due to their presence on the edge of the receiver housing and thus require careful placement for efficient performance. In addition, these designs, while providing an inexpensive case design often separate the transceiver electronic circuits from the antenna, thus requiring extra manufacturing steps in the form of lead wire insertion and soldering to bring the two elements to electric conjunction.
Another prior design, as disclosed in U.S. Pat. No. 4,975,711 to Kang-Hoon Lee and assigned to Samsung Electronic Company, provides for a slot antenna structure that is omni-directional by placing the antenna slots in orthogonal planes with coaxial cable connecting the slots together and to the receiver electronic circuits. Such a design is high in manufacturing costs due to the cable connections required during assembly. In addition, the design suffers from the proximity of the antenna structure to the receiver holding structure or body.
As has been discussed, the present state of antenna art does not provide for a small, flat, omni-directional antenna that is suitable for low cost, easily manufactured miniature tag transceivers. It is this deficiency that is addressed with the present invention.